The conventional optical encoder, shown in FIG. 1, is a blocking-type encoder. The encoder comprises a grating wheel 60, a light source 70, and a photo-detector 80. The light source 70 is arranged near the grating wheel 60 so as to emit light toward the grating wheel 60. The grating wheel 60 is centered on an axis 50 and is rotated by a driving means (not shown). The grating wheel 60 is provided with a plurality of slits 61 along the distal portion thereof. Moreover, there is an opaque section 62 arranged between two adjacent slits 61. The photo-detector 80 is placed at the proximity of the grating wheel 60 and comprises a photo-sensitive chip 81 to receive the light emitted from the slit 60.
The light emitted from light source 70 is chopped by slit 61 and opaque section 62 of grating wheel 60, and becomes a working beam when grating wheel 60 is rotated. The working beam is detected by photo-detector 80 and is converted to a sinusoidal signal on an oscilloscope. The signal is processed by a rectifier to form four signals (0,0), (0,1), (1,1), (1,0).
However, due to the wave nature of light, light emitted from two slits may interfere with each other, as shown in FIG. 2. This interference may cause a malfunction in photo-detector 80.
To eliminate the interference caused by the two adjacent slits, one may bring the photo-sensitive chip 81 of photo-detector 80 near slit 61. Another approach is to make the area of slit 61 larger than that of the photo-sensitive chip 81 (an increase of at least a third). However, the first approach makes fabrication complicated and difficult, while the second approach degrades resolution.
As shown in FIG. 3A, D represents the light source, C is the grating, A and B are the two slits, and S is the photo-detector, which moves along direction T. FIG. 3B shows the distribution of light intensity, where N.sub.1 and N.sub.2 represent average intensity distributions and H represents probability. As shown in FIG. 3B, N.sub.1 =(H.sub.1).sup.2 is the average intensity distribution detected by photo-detector S, when slit B is blocked. Moreover, N.sub.2 =(H.sub.2).sup.2 is the average intensity distributions detected by photo-detector S when slit A is blocked.
If the light passing through slits A and B does not interfere, the light intensity distribution detected by photo-detector S will be N.sub.12 =N.sub.1 +N.sub.2, as shown in FIG. 3C.
However, according to the principles of wave optics, the light intensity distribution detected by photo-detector S is that shown in FIG. 4, due to the interference between two slits. To eliminate this interference, one may route the light by a path which does not pass through the slits. Another method is to focus the light before it reaches the photo-detector. As shown in FIG. 5, the photo-detector can be placed between the lens and the focal point of the lens to eliminate the interference phenomenon.